Means to control cross talk



June-3o, 1931. A, @RFE 1,811,915

MEANS TO CONTROL CROSS TALK Filed June 26. 1928 Zo/Impedance 1 INVENToR l A i TORNEY Patented' `June v30, 1931 f UNITED STATES PATE-.Nn OFFICE lALLEN CARPE, F NEW YORK, N, Y., ASSIGNOR T0 AMERICAN TELEPHONE ANI) TELE- GRAPH COMPANY, A CORPORATION 0F NEW YORK `'.MELANS TO CONTROL CROSS TALK Application led .Tune 26,

This invention relates to transmission circuits, and more particularly to-arrangements for reducing cross-talk or interference upon such circuits.

In designing telephone transmission circuits With respect to cross-talk limitations, there are two factors to be taken into consideration,'-iirst, the effect of cross-talk upon the intelligibility of the conversation While talking is taking place, and second, the absence of secrecy or the annoying effect of the cross-talk during gaps in the conversation. The latter factor determines the limiting condition, for the reason that the maximum cross-talk current which would be permissible without rendering the conversation unintelligible will be greater than the maxip mlnn cross-talk which may be permitted without rendering such cross-talk understandable or objectionable during gaps in the conversation.

It has heretofore been proposed to eliminate or reduce cross-talk or noise currents from external sources, during such intervals as conversation is not taking place, by providing switching arrangements for normally disabling the circuit for transmission purposes,

such disabling means being removed, however, under the control of voice currents when 3o conversation takes place.

It is proposed, in accordance with the present invention, to arrange the receiving apparatus of the carrier channels so that the receiving gain will be appreciably -less at small l, Referring to Fig. 1, L designatesl the connection extending from-the carrier receiving apparatus of one channel of the carrier 5 0 system to the main line upon which-the carinputs than at normal speech loads. This re- 1928. Serial No. 288,331.

rier channels are superposed. Included in the circuit L is a demodulator comprising a. vacuum tube D, so connected that its input circuit is supplied -with signaling -current through a transformer 10, the-output circuit being connected to an outgoing connection L1 through a transformer 11.' In the arrangement disclosed in F ig. 1, it is assumed that at the distant transmitter station the carrier is suppressed and only the side band is transmitted; Consequently, a ,source of carrier or beating frequency O is provided and is associated with the input of the demodulator through the transformer 12.

In order to control the effectiveness of the demodulator for currents of diiferent amplitudes, a resistance .13, shunted by a con. denser 14, is provided in the grid circuit. The normal biasing of the grid is determined by the usual grid battery 15. The potential. applied to the grid by the battery 15 is such that when only the carrier current from the source O is superposed on the grid, the potential yfrom the source 15, when added to the negative potential trapped upon the grid by reason of the .operation of the resistancecapacity combination 13- 14, will be just sufficient to cause the demodulator to operate on the straight or amplifying part of its characteristic. Under these conditions, the tube D is practically inefective as a demodulator, as the second power term of the current in its output is either notv present or is only present in avery minor degree.

When va signal band of very small am litude is received, as will be the case w en noise currents or cross-talk currents are transmitted into the circuit L, the trapped negative potential upon the grid is but very little increased, and the tube D still remains quite 'ineffective as a demodulator. When, however, signal bands of large amplitude, such as correspond to voice signals, are impressed upon the grid circuit, the eife'ct of the resistance-capacity combination 13-14 will be such as to trap a very much larger negative potential upon the grid and thereby biasthe grid suciently. negative to cause it to operate in the region of its lower knee.v Therefore, a. very large second power term appears in the output circuit, and the tube D becomes quite eifective as a demodulator.

The time 'constant of the resistance-capacity combination 13-14 should be such as to pre vent fluctuations of gain at frequencies normal occurring in the audio frequency band, bu hould allow the gain to change suliiciently rapidly so that no appreciable portion of a syllable will be lost when the talker starts to speak, and so that the gain will drop off rapidly at the end of a period of transmission. A time` constant of the, order of 0.005 to 0.01 second will generally be found satisfactory.

' Fig. 2 shows a modified arrangement in which the modulator comprises twovacuum tubes connected in a push-pull circuit. The side band from the circuit L is applied tothe common path of the grid circuits of the two tubes through a transformer 10, so that components corresponding to the undetected side band frequencies will be suppressed from the output circuit of the demodulator. The carrier frequency source O is so connected to the grid circuits of the two tubes o f the demodulator that the grids of the two tubes will be effectively in series with respect to the carrier current. Consequently, the' carrier current is not suppressed from the output circuit of the modulator. As before, the re- 'sistance-capacity'combination-l3-14 and the grid biasing battery 15 are so proportionedthat .with no side band present and only the carrier current being applied, the demodulator tubes will be operated on the straight or amplifying parts of their characteristics and will be consequently very ineffective as demodulators.

When a side band corresponding to weak signaling currents, such as noise or cross-talk, is received, the negative charge trapped by theresistance-capacity combination will bias the "grids but little further negative than under the condition above referred to, and hence the demodulator will be ineffective to produce audible currents corresponding to the noise or cross-talk. When side bands corresponding to strong signals, such as voice currents, are received, however, the negative charge trapped upon the grids of the devvmodulator will be' sufficiently large to shift the point of operation of the tubes to the neighborhood of the lower knee of the characteristic curves, so that the demodulator betion frequently employed in connection with the detectors of radio receiving sets. In such detectors, the resistance and capacity o f the grid-leak combination are so proportloned -means of the must-permit the capacity to discharge withV suicient rapidity so that the negative char-ge upon the rid will vary with the envelo e of the high requency wave; in other wor s, so that the charge on the grid will vary with the voice frequency wave. In the case of the demodulators herein described, however, the time constant is much larger so that the charge does not vary with therise and fall of the individual voice waves. On the contrary, the chargeis determined by the general'level of a train of voice waves and merely varies with changes in the amplitude or volume of the speech as the speaker raises and lowers his voice in talking. It is this adjustment ofthe resistance-capacity 'combina-l tion that enables the demodulators herein described to discriminate as regards their gain with respect to voice signals and cross-talk. A grid-leak arrangement, such as is employed in a usual radio detector, does not have this this instance, the circuit Lis connected to an amplifier comprising two vacuum tubes A and'A connected 'in parallel, so that both tubes are interposed between the circuit L and the circuit L1. A Transformers 12 and 12',

Which'may consist of windings on a common core, are 'used to cou le the .circuit L to the inputs of the tubes and A', andlikewise,

a common core, are used to connect the 'outgoing circuit L1 tothe two tubes. The. secondary winding of one .of the output transformers, for example 11', is reversed with respect to the secondary of the other transformer, with the result that when the two vacuumtube circuits are properly balanced, no transmission takes place between circuit L and circuit L1.

The normal id potentials are supplied by battery 15. These potentials are small, of the same order of magnitlide' as the interfering voltages which it is desi-red t0 suppress. A' resistance-capacity combination 13--14, similar to that reviously described, is included in the grid) circuit of the,

upper tube A. The function of this resistance-capacitycombination is to increase the grid bias of the upper tube whenever the input voltage exceeds that of the biasing .battery 15. This change in ridbias will unbalance the circuit, due to t e resultant change in plate impedance, and a differential output transformers 11 and 1l', which may also have will be obtained. Obviously, for weak c urrents, such as noise or cross-talk currents received in the circuit L, which do not, during their positive half cycles exceed in potential Y the voltage due to the battery 15, the amplibeing biased differently from the lower tube,

so`that the system is unbalanced and the amplifier transmits effectively. In order that the change in plate impedance may result in an effective differential output, the impedance of the circuit L1 should not be too large com ared with the plate impedance.

Tlie fact that the grids go positive with each positive cycle, as above described, will result in distortion in case the external impedance of the input circuit is large. In order to substantially eliminate and reduce distortion, the circuit L should be made of low impedance. Each time the grid of a tube goes positive, some current ilows between the grid and filament. So long as the grid is negative, no current can low,and the grid-filament impedance is infinite. As -soon as curr'ent does flow, however, the rid-filament impedance is reduced to some ite value. The

potential drop between the grid and. filament,

of course, depends upon the grid-filament impedance. When the grid-filament impedance is infinite, the full voltage ofthe signal applied to the circuit appears between the grid and the filament. When, however, the impedance between the grid and the filament ldro s to a finite value, the drop between the gri and filament depends upon the ratio be-v tween the grid-lament impedance and the impedance looking into the circuit L. The lower the impedance looking into the circuit L is made, the greater will be the proportion of the total si al voltage which appears -as` the potential rop between the grid and ilament. Consequently, by keeping the impedance ofthe circuit L low, the distortion due tochange inthe impedance of tl..e gridfilament circuit may be very much reduced, and, in fact, practically eliminated.

Animproved circuit, embodying the same principles, is illustrated in Fig. 4. The circuit is generally similar to that of Fig. 3, but diiers in the fact that a separate. grid polarizing battery is provided foreach ampliiier tube. The potential of the polarizing battery for the grid of tube A will be thatnormally used for amplifier tubes. The

battery 15 for polarizing the grid of the upper tube A, however, will Vbe such as to produce a smaller grid bias, and, in fact, the

grid bias of the upper tube may be zero. In order to balance the two amplifying circuits so as to suppress weak signaling currents and prevent their transmission'to the .circuit L1, the secondary winding of one of the output transformers is reversed with respect to that of the other, as in the case of Fig. 3. Furthermore, due to the diderence in the biasing potentials applied to the grids of the two tubes A and A', the normal output impedances of the two tubes will be different; and, consequently', the primary winding' of the output transformer 11 of the upper tube is made of fewer turns than the primary winding of the output transformer 11 of the lower tube.

Under these conditions, the amplifier cir- 15, the resistance-capacity combination 13 14 will have no effect to change the grid bias of the tube A, and hencethe amplifier system will balance out such signals. When signa-ls of large amplitude, such as thosey due to .voice currents, are received,- 'such signals will be transmitted, without distortion,

through the lower amplifier A', due to the fact that Vthe bias on the gridrof this tube is a normal amplifier bias. Such signals, when applied to the upper tube, will, as to .their positive half cycles, exceed the voltage of the biasing battery 15. Consequently, the` resistance capacity combination .13 14 comes into play-to drive the grid of the upper tube more negative and thereby unbalance the circuit, with the result that effective transmission takes place between the circuit Land the circuit L1.

It should be noted that the impedance looking into the circuit L1 shouldnot be.V

'too high as compared .With the output impedance of the upper tube A, so that the decrease with an increase in the plate vimpedance of the tube A, when the input is greater in amplitude than the voltage of the biasing battery 15.

Fig. 5 shows a slightly different arrangement, in which the control is eiected by means of a resistance-capacity combination in a grid circuit of the oscillating tube which supplies the oscillations to a demodulator. Here the circuit L is connected through fa transformer 12to the input of a balanced detector arrangement comprising tubes D and D whose output is connected through a trans ormer 11 to the outgoing circuit L1. The .beating carrier frequency. is supplied by a vacuum tube oscillator O, which is connected to the common branch of the grid combination 13e-,14 is connected in the grid circuit of the oscillator O, and the grid circuit of the oscillator is also associated with the incoming circuit L through a transform- I. @transmitted through the detector path, and

viceversa 55,- cluded in the circuit L between the connection `Under normal`conditions, when no sideO band is being-f'receivedaimthe circuit L, the

grid potential of .the oscillator-Q is. deter- A mined by the grid battery Eg and the amount of carrier oscillations delivered to the demod# ulator will be determined by the constants of the circuit and of the transformer 16. The circuit may be so adjusted that under Athese conditions the oscillations supplied to the de? modulator will be of such amplitude as to render it relatively inelfective ilrthe production of signals. When a voice band corre'- sponding'to a large signaling current, such as a voice current, is being received, some of this energy is impressed through the transformer 17 upon the grid circuit of the oscillator O.

The effect of the resistance-capacity combination 13-14 is to trap a large negative potential upon the grid, which shifts its bias in avnegative direction and thus alters the amplitude of the carrier oscillations produced by it. By proper adjustment of the circuit, the demodulator may be caused-to operate more eliiciently with this different amplitude of carrier than was the case when no signal voltage was impressed upon transformer 17.

If the side band representing a signal of small amplitude, such as noise or cross-talk, is being received, the e'ect of thecapacity combination 1 3--11` is to drive the biasing potential of the grid of the oscillator but little more negative than under no-loadv conditions, so that the amplitude of the carrier oscillations is little changed and the demodulator remains relatively ineffective. Hence, for noise or cross-talk currents, the detected output current is very small,'but in the case of signal currents of large amplitude, the corresponding detected currents appearing in the circuit L1 will be large.

If desired, a delay circuit F may into the transformer 17 and the connection to the transformer 12 to permit the negative `charge to build up on the grid of the tube O before the voice wave arrives at the detector,

thus preventingany clipping or discriminar tion against the initial parts of the voice wave.

from those illustrated, witliout departing from the spirit of the invention as defined in the following claims.

What is claimed is:

1. In a transmission system, a transmission circuit, a vacuum tube device associated with saidy circuit, and means to render the gain of said vacuum tube device appreciably less in response to small received current than in response to normal speech signals, said means comprising a capacity shunted by a resistance in the grid circuit of said tube and cooperating with the rectiying) action of the grid filament path in the tu e to produce a oneway biasing potential in the grid circuit, said resistance and condenser being so proportioned as to change the bias of said grid in accordance with the amplitude of the received current.

2.A In a transmission system, a transmission circuit, a push-pull translating circuit asso ciated with said transmission circuit, said push-pull circuit being normally substantially ineffective to translate signaling currents received from said transmission circuit, a vacuum tube associated with said push-pull circuit, and means to render said push-pull circuit less eifective in response to small received current than in response to normal speech signals, said means comprising a capacity shunted by a resistance in the grid circuit of said tube and so proportioned as to change the bias of said grid in accordance with the amplitude of the received current.

3. In a transmission system, atransmission circuit, a vacuum tube device associated with said circuit, means to apply a normal steady.

biasing potential to the grid of a tube of said device, means to render the gain of said receiver appreciably less in response to small received current than in response to normal speech signals, said last mentioned means .comprising a capacity shunted by a resistance in the grid circuit of said tube and cooperating with the rectifying action of the grid filament path in the tube to produce a one-way` biasing potential in the grid circuit, said resistance and condenser being so proportioned 'as to change the bias of said grid in accordance with the amplitude of the received current.

4. In a transmission system, a transmission circuit, an amplifying arrangement comprising a pushpull circuit associated with said transmission circuit and'normally so balanced with respect to the transmission circuit as to be substantially ineffective to transmit signaling currents receivedxfrom said transmission circuit, and means to produce an unbalance of the push-pull circuit in accordance with the lamplitude of received signaling current,

whereby said amplifying arrangement'wlll be f more effective in'response to normal volce cury rents than in response to weak signaling currents. 0

5. In a transmission system, a transmisslon circuit, an amplifying arrangement associated with said transmission circuit and comprising a pair of vacuum tubes arranged in a. .push-pull circuit, said push-pull circuit beingv l normally so balanced as to be substantially ineffective to transmit signaling currents received from said transmission circuit, and meansY to produce an unbalance of the pushpull circuit in accordance with the amplitude of received signaling current, said means com- Y prising a capacity shnnted by a resistance in the grid circuit of one of said tubes andso proportioned as to change the bias'of the grid of said tube in accordance with the ampli-' tude of the received current, whereby said amplifying arrangement will be more eifective in response to normal voice currents than in respense to weak signaling currents. i In testimony whereof, I have signed my name` to this specification this 21st day of June, 1928.

ALLEN CARPE. 

